Is there a dua to change someone?

Thanks for the replies. I will definitely look into it. Is there also something one can recite to increase love and peace in the house (Al-Wadood, if memory serves me correct?)

Unfortunately, it's not just a teenage thing, she is genuinely upset with my mom. It kills me to see my mom get abused (mentally) by my sister and always feel lonely. It is my greatest wish on earth that the love/relationship between them is restored.

Thanks,
Ehsan

First thing first: Allah will ALWAYS answer/grant our du'a. Our du'a may not be answered immediately or in the form that we prefer, though. Or it may be reserved for our syafaat during judgement day.

I don't know of any special du'a to increase love and peace in the house or to change someone.
However, from hadiths, we know that there are times and places where Allah is nearest to us and hence it is mustahhab to make supplications:
after shalat fard, during prostration (especially the last prostration), during sitting between two sujood at the second to last rakaat, when it is raining, shalah tahajjud during the last third of the night (presumably between midnight till fajr), when we are breaking fast (so do voluntary fastings, monday-thursday or three days in the middle of the hijriah calendar month), etc.

So, you can ask Allah and make du'a in those situations.
Other things to mind to make our du'as answered and succesful:
- be in the state of wudoo and in clean clothing,
- prior to make du'a, invoke the beautful names of Allah (eg. Ya hayyu Ya Qayyum, Ya Arhamar raahimin, Dzulzalali wal ikram, etc etc), and also send salam and shalawat to the prophet SAW
- make dua as sincerely as possible, be in extreme humble, in a state of hope and fear, pour your heart of, cry if you must

Also please be PATIENT (sabr), and even if your dua is not answered immediately, do not give up on hope and do not stop making the dua.
I will give you an example from my experience: When I was "lost" in my ways, my mother made duas constantly that Allah guide me back to the straight path. Her dua was not answered overnight, but she kept making the duas for YEARS before I came back to the fold of Islam and the straight path Alhamdulillaah.
So have a great deal of sabr and trust in Allah that your situation will get better Insha Allah.

ASA

I am going to try everything. I have faith in allah and I will hope that things get better.